Patent Application
20130308716
The present invention relates to wireless communication technologies.
The reception characteristics of terminals positioned at a cell edge in a cellular system or the reception characteristics of reception devices in wireless communication systems may be significantly degraded due to the effects of interference (undesired signals) coming from an adjacent interference source (an adjacent cell or an adjacent wireless LAN system), the wireless systems using the same frequency band and their communication possible areas overlapping each other. Such wireless communication systems are, for example, a plurality of wireless LAN systems used in rooms that are next to one another. Interference Alignment (hereinafter referred to as “IA”) has been proposed as an interference reduction method that is effective in the case where there are a plurality of transmission sources that use the same frequency band (see NPL 1 below).
When IA is used, devices on a transmission side are controlled so as to collaborate with one another such that the (vector) directions of equivalent channel vectors of interference components are brought into alignment at the time of reception, the interference components coming from a plurality of transmission devices, which are interference sources. Consequently, even in the case where the number of interference signals that have arrived at a reception device is greater than the number of interference signals that may be eliminated, a desired signal may be extracted from a reception signal. Here, a value called the degree of freedom is used as a standard for determining the number of interference signals that may be eliminated in a reception device, the degree of freedom being determined in accordance with the number of resources, examples of the resources being time resources, frequency resources, and space resources (antennas) with which the reception device may receive signals. For example, the number of space resources, that is, the number of antennas is described as an example. When a reception device has three receive antennas, two interference signals may be eliminated and one desired signal may be extracted. The degree of freedom in this case is two. In the case where the degree of freedom is large, the larger number of interference signals may be eliminated. In this way, the degree of freedom is a value determined by the number of resources used. The resources used are not limited to the above-described space resources. Even in the case where a plurality of time resources or frequency resources are used, a similar relationship is obtained.
Here, as illustrated in
[Math. 1]
y
i
=H
ii
v
ii
x
ii
+H
ij
v
ij
x
ij
+H
ji
v
ji
x
ji
+H
jj
v
jj
x
jj (i≠j) (1)
As expressed by Equation (1), four signals (two desired signals and two interference signals) arrive at a reception device. Thus, in order to extract each of the desired signals from the reception signal one by one, the rest three signals need to be eliminated as interference. Thus, the degree of freedom needs to be three. However, each reception device illustrated in
Under such circumstances, when IA is applied with which the vectors of interference signals coming from transmission devices are brought into alignment, there is a relationship Hiivji=kHijvjj between the equivalent channel vectors of the interference signals, which are the third and fourth terms of Equation (1). Here, k is an arbitrary scalar but is a value determined in accordance with the transmission power of each transmission device and the like in an actual system. Here, for the sake of simplicity of the description, k=1, that is, Hiivji=Hijvjj. In order to achieve such a relationship, it is necessary to adjust transmission weight vectors, each of which is used in a corresponding transmission device. This may be realized, for example, by determining a transmission weight vector vji used in a transmission device i and then by determining a transmission weight vector vj used in a transmission device j from vji, Hij+Hiivji (+ represents a generalized inverse). Here, vji, which is determined first, is an arbitrary vector and may be set to a vector, for example, such as vji=[1 1]T. Moreover, the order in which the transmission weight vectors vji and are determined may be reversed. Note that it is necessary to perform setting such that two transmission weight vectors used in one transmission device are not parallel with each other (such that the inner product is not zero. For example, vii≠avji (a is an arbitrary scalar).)
In this way, in the case where the equivalent channel vectors of interference signals are brought into alignment at the time of reception, Equation (1) is changed to the following equation.
[Math. 2]
y
i
=H
ii
v
ii
x
ii
+H
ij
v
ij
x
ij
+H
ji
v
ji(xji+xii) (2)
Equation (2) indicates that two desired signals and one interference signal are received. Thus, it is understood that each reception device may extract each desired signal in accordance with the degree of freedom (here, two) of the reception device.
For each desired signal, a reception signal like this and a reception weight vector for extracting the desired signal are multiplied together in a reception device. Here, reception weight vectors uii and uij for completely eliminating interference and extracting desired signals xii and xij, respectively, satisfy the following equation.
[Math. 3]
u
ii
[H
ij
v
ij
H
ii
v
ji]=0
u
ij
[H
ii
v
ii
H
ii
v
ji]=0 (3)
In Equation (3), for example, the first equation indicates that the reception weight vector uii is a vector orthogonal to the vectors Hijvij and Hiivji. Such a vector, the vector uii, is the complex conjugate transpose of a right-singular vector corresponding to a singular value that is zero from among right-singular vectors obtained by performing singular value decomposition (SVD: Singular Value Decomposition) on a matrix [HijvijHiivji]H. That is, uii may be obtained from uii=e2H by using e2 of Equation (4). Note that F and E are unitary matrices and D is a diagonal matrix with nonnegative real numbers on the diagonal.
[Math. 4]
[Hijviivji]HFDEH=FD[e1e2]H (4)
Similarly to uii, uij may be determined to be the complex conjugate transpose of a right-singular vector corresponding to a singular value that is zero from among right-singular vectors obtained by performing SVD on a matrix [Hiivii Hiivji]H.
In the case where the above-described transmission weight vectors are used, Equation (2) is changed to Equation (5). Each of the desired signals xii and xij may be extracted while completely eliminating interference.
[Math. 5]
u
ii
y
i
=u
ii
H
ii
v
ii
x
ii
+u
ii
H
ij
v
ij
x
ij
+u
ij
H
ii
v
ji(xji+xjj)=uiiHiiviixii
u
ij
y
i
=u
ij
H
ii
v
ii
x
ii
+u
ij
H
ij
V
ij
x
ij
+u
ij
H
ii
v
ji(xji+xjj)=uijHijvijxij (5)
In this way, even in the case where the number of interference signals that have arrived at a reception device is greater than the number of interference signals that may be eliminated, desired signals may be extracted from a reception signal by applying IA with which the vectors of interference signals coming from transmission devices are brought into alignment. That is, IA makes it possible to perform transmission while making the most use of the degree of freedom.
In the case where IA is used, it is necessary to feed back channel information (CSI: Channel State Information) obtained in a reception device to a transmission device and to determine a transmission weight vector in accordance with the CSI that has been fed back. However, in the case where a reception device or a transmission device moves or in an environment in which objects around a reception device or a transmission device move, changes may occur in channels between the time when a reception device performed estimation and the time when signals are transmitted using a transmission weight vector (a CSI error occurs). The channel matrix obtained at the time when CSI estimation is performed is denoted by H. For example, as described above, it is assumed that Hiivji=kHijvjj (k is an arbitrary scalar) are vectors (equivalent channel vectors) of interference signals that are brought into alignment by using IA. In this case, even in the case where the channel matrices obtained at the time of CSI estimation are changed to channel matrices H′ at the time of data transmission, when Hii′vji=kHij′vjj is satisfied, the vectors of interference signals are brought into alignment. Thus, interference may be completely eliminated by using existing reception weight vectors. That is, as expressed by Equation (3), the complex conjugate transpose of a right-singular vector corresponding to a singular value that is zero should be used from among right-singular vectors obtained by performing SVD on [Hij′vij Hii′vji] H.
However, in general, changes in channels are independent of one another, it is almost impossible that Hii′vji=kHij′vjj is satisfied. In such a case, the vectors of interference signals are not brought into alignment at the time of reception. Thus, it is impossible to eliminate interference even when Equation (3) or the above-described reception weight vectors are used, and there is an issue in that the reception characteristics are significantly degraded. Such a CSI error occurs not only due to changes in channels but also due to a CSI-estimation error caused by the effects of thermal noise added at a reception device, a quantization error that occurs when quantization is performed for feedback, or the like. In any of the cases, the reception characteristics are degraded.
It is an object of the present invention to suppress degradation of the reception characteristics in a system that uses IA even under the circumstances in which a CSI error occurs.
The present invention is a wireless communication system including a plurality of transmission devices, each of which transmits signals resulting from precoding performed for a plurality of resources, and a reception device that receives at least one desired signal and a plurality of undesired signals, the number of which is greater than or equal to the degree of freedom that the plurality of resources have. The at least one desired signal and the plurality of undesired signals have been transmitted from the transmission devices. The plurality of resources is the unit of precoding. At least one of the plurality of transmission devices transmits signals on each of which precoding has been performed such that equivalent channel vectors of the plurality of undesired signals in the reception device are made to be orthogonal to a reception weight vector used in the reception device. The reception device estimates equivalent channel vectors of the plurality of undesired signals, calculates a reception weight vector by using the estimated equivalent channel vectors of the plurality of undesired signals, and extracts a desired signal by multiplying a reception signal received using the plurality of resources and the calculated reception weight vector together. The plurality of resources is the unit of precoding.
A desired signal is extracted by multiplying a reception data signal and a reception weight vector as described above together. Thus, the degradation of characteristics due to the effects of a CSI error may be reduced even in the case where a CSI error occurs in a system in which IA is used. Note that, the present invention may be applied not only to IA performed using a plurality of space resources but also to IA performed using a plurality of time resources or frequency resources.
In addition, the present invention is a reception device to which signals are transmitted, each of which results from precoding performed for a plurality of resources in at least a part of a plurality of transmission devices such that equivalent channel vectors of undesired signals in the reception device are made to be orthogonal to a reception weight vector used in the reception device, and that receives at least one desired signal and a plurality of undesired signal, the number of which is greater than or equal to the degree of freedom that the plurality of resources have. The plurality of resources is the unit of precoding. Equivalent channel vectors of the plurality of undesired signals are estimated, a reception weight vector is calculated by using the estimated equivalent channel vectors of the plurality of undesired signals, and a desired signal is extracted by multiplying a reception signal received using the plurality of resources and the calculated reception weight vector together. The plurality of resources is the unit of precoding.
In addition, the present invention is a transmission device that transmits signals, each of which results from precoding performed for a plurality of resources in at least a part of a plurality of transmission devices such that equivalent channel vectors of undesired signals in a reception device are made to be orthogonal to a reception weight vector used in the reception device, the reception device receiving at least one desired signal and a plurality of undesired signal, the number of which is greater than or equal to the degree of freedom that the plurality of resources have. The at least one desired signal and the plurality of undesired signals have been transmitted. The plurality of resources is the unit of precoding. A channel state information estimation signal is transmitted by using resources that are orthogonal to one another with respect to the plurality of transmission devices in order to estimate the equivalent channel vector of the desired signal and the equivalent channel vectors of the plurality of undesired signals in the reception device. The channel state information estimation signal has been used in the precoding.
The present specification contains the contents of the specification and/or drawings of Japanese Patent Application No. 2011-003047 that was filed in the Japan Patent Office and to which the present application claims priority.
According to the present invention, the degradation of the reception characteristics may be suppressed in a system that uses IA even under the circumstances in which a CSI error occurs.
In the following, wireless communication technologies according to embodiments of the present invention will be described with reference to the drawings.
First, in the case where IA is used in a system illustrated in
As illustrated in
[Math. 6]
y
i
=H
ii
v
ii
x
ii
+H
ij
v
ij
x
ij
+H
ij
v
ij
+H
ij
v
jj(kxji+xjj) (6)
In contrast, in the case where there is a CSI error, the reception signal yi is expressed as the following Equation (7). Note that H′ denotes a channel matrix to which the channel matrix obtained at the time of CSI estimation has been changed.
[Math. 7]
y
i
=H′
ii
v
ii
x
ii
+H′
ij
v
ij
x
ij
+H′
ii
v
ji
x
ji
+H′
ij
v
jj
x
jj (7)
In this way, in the case where there is a CSI error, even when the transmission weight vectors of transmission devices are adjusted in a collaboration manner so as to satisfy HiiVji=kHijvjj, the vectors of interference signals are unable to be brought into alignment at the time of reception and circumstances occur in which the degree of freedom is insufficient. In this case, it is impossible to completely eliminate interference. In order to reduce the effects of interference as much as possible, reception weight vectors as described below need to be used.
First, a reception weight vector uii for extracting a desired signal xii is the complex conjugate transpose of a right-singular vector corresponding to the smallest singular value from among right-singular vectors obtained by performing SVD on the following matrix.
[Math. 8]
[H′ijvijH′iivjiH′ijvjj]H (8)
That is, uii is obtained from uii=e3
[Math. 9]
[H′ijvijH′iivjiH′ijvjj]H=FiiDiiEiiH=FiiDii[e1
The matrix shown on the left-hand side of Equation (9) (or Equation (8)) is the complex conjugate transpose of a matrix in which signals other than a desired signal (here, xii) to be extracted are arranged, that is, in which all the equivalent channel vectors of interference signals are arranged. The vector e3
Moreover, similarly to uii, a reception weight vector uij for extracting a desired signal xij is the complex conjugate transpose of a right-singular vector corresponding to the smallest singular value from among right-singular vectors obtained by performing SVD on the following matrix.
[Math. 10]
[H′iiviiH′iivjiH′ijvjj]H (10)
That is, uij is obtained from uij=e3
[Math. 11]
[H′iiviiH′iivjiH′ijvjj]H=FijDijEijH=FijDij[e1
In the case where the reception weight vectors as described above are used, Equation (7) is changed to the following Equation (12) and it is impossible to completely eliminate interference. Even though an interference signal denoted by z remains, the desired signals xii and xij are extracted while minimizing interference.
[Math. 12]
u
ii
y
i
=u
ii
H′
ii
v
ii
x
ii
+u
ii
H′
ij
v
ij
x
ij
+u
ii
H′
ii
v
ji
x
ji
+u
ii
H′
ij
v
jj
x
jj
=u
ii
H′
ii
v
ii
x
ii
+z
ii
u
ij
y
i
=u
ij
H′
ii
v
ii
x
ii
+u
ij
H′
ij
v
ij
x
ji
+u
ij
H′
ij
v
jj
x
jj
=u
ij
H′
ij
v
ij
x
ij
+z
ij (12)
Moreover, when this is expressed as a matrix operation, [uiiT uijT]Tyi is obtained. In the case where the phases of desired signals obtained in this way are also compensated, each of the reception weight vectors ui and uij a reception signal should be multiplied together, the reception weight vectors uii and uij being used to reduce interference. Then, each of (uiiHii′vii)H and (uijHij′vij)H and a corresponding one of the results should be multiplied together. That is, for each of the desired signals, the phase of the desired signal may be compensated by performing multiplication using the complex conjugate transpose of a vector obtained by multiplying a corresponding reception weight vector and the equivalent channel vector for the desired signal together. Furthermore, the amplitude of the desired signal may also be compensated by dividing a signal obtained as a result of weight multiplication by the square of the norm of the signal.
Moreover, here, as expressed by Equation (9) or Equation (11), the SVD is performed on the complex conjugate transpose of a matrix in which the equivalent channel vectors of interference signals are arranged, and thus the complex conjugate transpose of a right-singular vector corresponding to the smallest singular value is used as a reception weight vector. However, the SVD may be performed on a matrix in which the equivalent channel vectors of interference signals are arranged. In this case, the complex conjugate transpose of a left-singular vector corresponding to the smallest singular value is used as a reception weight vector.
Even in the case where the IA is applied by using the above-described reception weight vectors but the vectors of interference signals are not completely brought into alignment due to the effects of a CSI error and the interference signals, the number of which is greater than the degree of freedom that a reception device has, are received, it is possible to minimize the effects of interference and to extract desired signals from a reception signal. Thus, the degradation of reception characteristics may be reduced in a system in which IA is used even under the circumstances in which a CSI error occurs.
In the transmission device illustrated in
Here, it is necessary to make pilot signals, which are to be transmitted from the transmit antennas 15-1 and 15-2, be orthogonal to one another (or prevent from interfering with one another) in order to make a reception device estimate channels between the reception device and each of the transmit antennas 15-1 and 15-2 of each transmission device. Methods for making pilot signals be orthogonal to one another include a method in which pilot signals are made to be orthogonal to one another in the time domain, a method in which pilot signals are made to be orthogonal to one another in the frequency domain, a method in which pilot signals are made to be orthogonal to one another by using orthogonal codes, and the like. Any of the methods may be applied to the present invention. Here,
In addition, as described above, pilot signals may also be made to be orthogonal to one another in the frequency domain. In this case, it is desirable that each transmit antenna be configured to transmit a pilot signal in a corresponding one of sub-carriers in multicarrier transmission. This may be realized as follows. For example, there are four sub-carriers. In this case, in a sub-carrier 1, a pilot signal is transmitted from the transmit antenna unit 15-1 of the transmission device 1. In a sub-carrier 2, a pilot signal is transmitted from the transmit antenna unit 15-2 of the transmission device 1. In a sub-carrier 3, a pilot signal is transmitted from the transmit antenna unit 15-1 of the transmission device 2. In a sub-carrier 4, a pilot signal is transmitted from the transmit antenna unit 15-2 of the transmission device 2.
Furthermore, in the case where pilot signals are made to be orthogonal to one another by using orthogonal codes, the pilot signals to be transmitted from the transmit antennas and orthogonal codes are multiplied together, the orthogonal codes being different from each other and a pilot signal to be transmitted from each of the transmit antennas and a corresponding one of the orthogonal codes being multiplied together. A reception device is configured to separate the received pilot signal into pilot signals transmitted from the transmit antennas by multiplying the received pilot signal and, again, these orthogonal codes together and to estimate each channel matrix.
In this way, transmission device transmit pilot signals that are orthogonal to one another and a reception device estimates channel matrices in accordance with the pilot signals. The estimated channel matrices are fed back as CSI from the reception device to the transmission devices. Here, the CSI, which is fed back, is each of the channel matrices H expressed in Equation (6) described above. In the present embodiment, all the channel matrices are fed back to each transmission device. The CSI fed back from the reception device is received by the receive antenna unit 21 of the transmission device illustrated in
The channel matrices recovered by the reception unit 18 are input to the transmission weight calculation unit 17 and used to calculate a transmission weight vector. Here, in the case where IA is performed with respect to a plurality of transmission devices, it is necessary to adjust transmission weight vectors, which are to be used, for the transmission devices in a collaboration manner such that the directions of interference vectors are brought into alignment in the reception device. However, the present invention does not specify a method for calculating these transmission weight vectors, and any method may be used. For example, a method may be used in which v11 is determined by the transmission device (1) 1-1 and v22 is determined by the transmission device (2) 1-2, the transmission devices (1) 1-1 and (2) 1-2 exchange information regarding the determined transmission weight vectors, v21 is calculated from v21=kH11+H12v22 in the transmission device (1) 1-1, and v12 is calculated from v12=kH22+H21v11 in the transmission device (2) 1-2. Note that k is an arbitrary scalar and + represents a generalized inverse.
Here, v11 and v22 determined first may be arbitrary vectors; however, it is desirable that they be unitary vectors, considering limited transmission power. Alternatively, a determination method may be used in which v11 is set to a right-singular vector that is obtained by performing SVD on H11 and that corresponds to the largest singular value and v22 is set to a right-singular vector that is obtained by performing SVD on H22 and that corresponds to the largest singular value.
Alternatively, a determination method may be used in which two transmission weight vectors v11 and v21 are first determined by the transmission device (1) 1-1, the transmission device (2) 1-2 is notified of the information regarding the determined transmission weight vectors, v12 and v22 are determined in the transmission device (2) 1-2 by using the relationships v12=kH22+H21v11 and v22=kH12+H11v21 in accordance with the notified information. In this case, v11 and v21 that have been first determined may be arbitrary vectors that satisfy the relationship v11≠av21 (a is an arbitrary scalar); however, it is desirable that the vectors be orthogonal to one another in order to efficiently eliminate interference in the reception device. Alternatively, a determination method may be used in which v11 is set to a right-singular vector that is obtained by performing SVD on H11 and that corresponds to the largest singular value and v21 is set to a right-singular vector that is obtained by performing SVD on H21 and that corresponds to the largest singular value. Here, an example in which two transmission weight vectors are first determined in the transmission device (1) 1-1 has been described; however, in contrast, transmission weight vectors may be first determined in the transmission device (2) 1-2 and the transmission device (1) 1-1 may be notified of the information regarding the determined transmission weight vectors.
The calculation method in which transmission weight vectors are calculated in a collaboration manner is a mere example, and the present invention does not specify a calculation method for calculating transmission weight vectors but calculation of such transmission weight vectors is performed by the transmission weight calculation unit 17. Note that, as described above, the transmission weight vectors in the present embodiment include a transmission weight vector determined first and a transmission weight vector determined in accordance with the transmission weight vector determined first. The structure in which another transmission device is notified of a transmission weight vector determined first and the structure in which information regarding a transmission weight vector that is notified by the other transmission device is received are necessary. Thus, a transmission weight vector determined first by the transmission weight calculation unit 17 is input to the upper layer 10 and then modulated by the modulation unit 11 in a modulation method such as QPSK (Quadrature Phase Shift Keying), 16QAM (Quadrature Amplitude Modulation), or the like. D/A conversion is performed by the first D/A unit 13-1, and then the resulting signal flows via the first wireless communication unit 14-1 and is transmitted from the antenna unit 15-1. In the case where it is necessary to notify another transmission device of information regarding the determined transmission weight vector, notification is performed in this way. Note that, the present embodiment shows an example in which another transmission device is notified of a transmission weight vector only from one antenna. Moreover, similarly to the CSI fed back from a reception device, the information regarding a transmission weight vector that has been notified from the other transmission device is received by the receive antenna unit 21 and input to the transmission weight calculation unit 17 via the wireless communication unit 20, the A/D unit 19, and the reception unit 18. Here, an example is shown in which the information regarding a transmission weight vector is transmitted to or received from in a wireless manner; however, in the case where transmission devices are connected with each other in a wired manner similarly to base stations in a cellular system, transmission devices may be configured to perform notification of a transmission weight vector via a wired network.
With the above-described structure, transmission weight vectors may be calculated. Next, transmission of data signals by using calculated transmission weight vectors will be described. First, the transmission weight vectors calculated by the transmission weight calculation unit 17 (v11 and v21 in the transmission device (1) 1-1 and v12 and v22 in the transmission device (2) 1-2) are input to the transmission weight multiplication unit 12. A data signal, which has been input to the modulation unit 11 from the upper layer 10 and has been modulated, is input to the transmission weight multiplication unit 12 in addition to the above-described transmission weight vectors, and the data signal and a transmission weight vector are multiplied together in the transmission weight multiplication unit 12.
Moreover, a known pilot signal is input from the pilot signal generation unit 16 to the transmission weight multiplication unit 12. Similarly to a data signal, the known pilot signal and a transmission weight vector are multiplied together. This pilot signal is a signal that is necessary to calculate reception weight vectors, which are used in the reception device. In order to calculate the reception weight vectors, it is necessary to estimate equivalent channel vectors such as those expressed in Equation (8) or Equation (10). Thus, the pilot signal and the transmission weight vector, which is the same as that used for the data signal, are multiplied together and transmitted.
In this way, signals obtained by multiplying each of a pilot signal and a data signal and a transmission weight vector together in the transmission weight multiplication unit 12 are input to the first and second D/A units 13-1 and 13-2, and D/A conversion is performed. Thereafter, in the first and second wireless communication units 14-1 and 14-2, frequency conversion to the wireless frequency band is performed. Each of the resulting signals is transmitted from a corresponding one of the transmit antenna units 15-1 and 15-2. Here, the signal obtained by multiplying the pilot signal and the transmission weight vector together is used to calculate reception weight vectors, that is, to demodulate the data signal; thus, the signal is multiplexed in the same frame as the data signal.
Note that, in order to estimate equivalent channel vectors as expressed in Equation (8) or Equation (10), similarly to the pilot signals for CSI estimation, pilot signals need to be orthogonal to one another and transmitted such that the pilot signals are not interfering with one another. For example, in the case where pilot signals are made to be orthogonal to one another in the time domain, pilot signals are transmitted as illustrated in
Here, the case where pilot signals are made to be orthogonal in the time domain has been described; however, pilot signals may be made to be orthogonal not in the time domain but in the frequency domain. Alternatively, pilot signals may be made to be orthogonal by multiplying each pilot signal and a corresponding one of a plurality of different orthogonal codes together. A system in which IA is used and that does not cause a CSI error is a target system in the present invention. In such a system, since the directions of received interference vectors are brought into alignment, when equivalent channel vectors as expressed in Equation (3) may be estimated, reception weight vectors may be calculated. Thus, it is not necessary to estimate equivalent channel vectors of interference signals coming from all the interference sources. However, in the case where a CSI error occurs, in order to estimate all the equivalent channel vectors of interference signals coming from all the interference source as expressed in Equation (8) or Equation (10), it is necessary to make all signals be orthogonal to one another and transmit the resulting signals, the all signals being obtained by multiplying, together, each of the pilot signals and a corresponding transmission weight vector the same as that used for a data signal.
Since the transmission devices have a structure as described above, transmission using IA is possible. Moreover, in the case where a CSI error occurs, the equivalent channel vectors of interference signals may be estimated, the equivalent channel vectors of interference being necessary when the reception device calculates reception weight vectors with which degradation of characteristics due to the effects of the CSI error is minimized.
Next,
In the reception device illustrated in
In the channel estimation unit 37 to which the received pilot signals have been input, channel estimation is performed by using known pilot signals. When the channel estimation is performed by using the pilot signals for CSI estimation (see
In the transmission unit 39, to which the channel matrices estimated by using the pilot signals for CSI estimation have been input, the channel matrices are converted into a format in which the channel matrices may be transmitted. The converted channel matrices, which are digital signals, are converted into analog signals by the D/A unit 40. Thereafter, the analog signals flow via the wireless communication unit 41 and are transmitted from the transmit antenna unit 42 to transmission devices. By performing such processing, the channel matrices between each of the transmit antennas of the transmission devices and the receive antennas are estimated and the estimation results may be fed back as CSI to the transmission devices.
Moreover, equivalent channel vectors necessary for reception-weight-vector calculation according to the present embodiment are first extracted in the reception weight calculation unit 38, to which the equivalent channel vectors estimated by using the pilot signals for reception-weight-vector calculation have been input. A matrix such as a matrix expressed as Equation (8) or Equation (10) is constituted by using the extracted equivalent channel vectors. Then, reception weight vectors u (u11 and u12 in the reception device (1) 3-1 and u21 and u22 in the reception device (2) 3-2) are calculated by performing calculation (SVD) expressed as Equation (9) or Equation (11), the reception weight vectors being used to minimize the effects of interference occurring due to the effects of a CSI error. As described above, the reception weight vectors u may be calculated so as to compensate the phases and amplitudes of desired signals.
The reception weight vectors u calculated by the reception weight calculation unit 38 in this way are input to the reception weight multiplication unit 34, and the data signal input from the signal separation unit 33 and the reception weight vectors u are multiplied together. As a result of this multiplication, signals as expressed in Equation (12) or signals expressed in Equation (12) including the desired-signal components, the phases and amplitudes of which have been also compensated, are obtained and these signals are demodulated by the demodulation unit 35 and input to the upper layer 36.
Since the reception device has such a structure, in the case where a CSI error occurs in a system using IA, the equivalent channel vectors of interference signals coming from all the interference sources may be estimated and the reception weight vectors for minimizing degradation of characteristics due to the effects of a CSI error may be calculated. Moreover, the channel matrices for the transmit antennas of each transmission device are estimated and may be fed back as CSI.
The desired signals are extracted by multiplying the received data signal and the reception weight vectors together as described above. Thus, even in the case where a CSI error occurs in a system using IA, the degradation of characteristics due to the effects of a CSI error may be reduced. In addition to this, there is a method for calculating reception weight vectors for reducing degradation of characteristics. For example, in the case where a reception device may know vectors (equivalent channel vectors) of interference signals that are supposed to be brought into alignment when a CSI error has not occurred, reception weight vectors may be calculated using the equivalent channel vectors of interference signals that are supposed to be brought into alignment. Specifically, instead of performing, for example, Equation (8), the complex conjugate transpose of a right-singular vector corresponding to the smallest singular value (zero) from among right-singular vectors obtained by performing SVD on [Hij′vij Hiivji]H may be used as a reception weight vector. Here, H′v represents an equivalent channel vector in the case where a CSI error occurs and Hv represents an equivalent channel vector in the case where no CSI occurs.
Moreover, a vector at the midpoint of equivalent channel vectors of interference signals that are not brought into alignment due to a CSI error may be calculated, and a reception weight vector may be calculated using the vector at the midpoint. Specifically, for example, instead of performing Equation (8), the complex conjugate transpose of a right-singular vector corresponding to the smallest singular value (zero in this case) from among right-singular vectors obtained by performing SVD on [Hij′vij (Hii′vji+Hij′vjj)/2]H is used as a reception weight vector. Note that, here, both vectors (Hii′vji and Hij′vjj) used to calculate the vector at the midpoint are equivalent channel vectors for signals that are not desired signals. Although Hij′vij is treated as interference when xii is extracted, Hij′vij is actually an equivalent channel vector of xij, that is, a desired signal. Thus, Hij′vij is not used to calculate a vector at the midpoint. In this way, a method in which a vector at the midpoint of the equivalent channel vectors of interference signals is used to calculate a reception weight vector is significantly effective as a method for reducing degradation of characteristics due to the effects of a CSI error, in the case where a leading cause that makes a CSI error occur is noise added to pilot signals in a reception device.
Furthermore, a reception weight vector may be calculated using an equivalent channel vector whose norm (size) is the largest from among the equivalent channel vectors of interference signals that are not brought into alignment due to a CSI error. Specifically, for example, in the case of |Hii′vji|2>|Hij′vjj|2, instead of performing Equation (8), the complex conjugate transpose of a right-singular vector corresponding to the smallest singular value (zero in this case) from among right-singular vectors obtained by performing SVD on [Hij′vij Hii′vji]H is used as a reception weight vector. A larger interference component may be completely eliminated by using such a reception weight vector and thus the degradation of characteristics due to a CSI error may be reduced.
Moreover, calculation of reception weight vectors according to the present embodiment may be applied not only to the system having a structure illustrated in
Furthermore, in the case where another transmission device is added and a transmission device 4 transmits a signal x4 by using a transmission weight vector v4 to a reception device 4, when a CSI error occurs, three interference signals H2′v2x2, H3′v3x3, and H4′v4x4 arrive at the reception device 1. Even in such a case, the effects of interference may be reduced by calculating a reception weight vector by using a similar method. Specifically, it is desirable that the complex conjugate transpose of a right-singular vector corresponding to the smallest singular value from among right-singular vectors obtained by performing SVD on [H2′v2 H3′v3 H4′v4]H be used as a reception weight vector. Moreover, in the case where the SVD is performed on [H2′v2 H3′v3 H4′v4], the complex conjugate transpose of a left-singular vector corresponding to the smallest singular value may be used as a reception weight vector.
In this way, in the case where the number of interference sources is increased, the effects of interference may be reduced by using reception weight vectors calculated in accordance with equivalent channel vectors of interference signals, the equivalent channel vectors being obtained after changes in channels have occurred.
Next, a second embodiment of the present invention will be described with reference to the drawings.
In the first embodiment, the reception weight vectors for minimizing interference occurring due to a CSI error under circumstances in which a CSI error occurs in a system in which IA is used, have been described as an example. The reception characteristics of a reception device depend not only on interference but also on thermal noise within the reception device. Thus, in contrast to the case where reception weight vectors obtained by considering only interference are used, the characteristics may be improved by using reception weight vectors obtained by considering both interference and thermal noise. In the present embodiment, a reception weight vector obtained by considering not only interference occurring due to a CSI error but also thermal noise within a reception device will be described. Specifically, the system illustrated in
As described above,
In such a system, in the case where no CSI error occurs, a reception signal y1 in the reception device 1 is expressed as the following equation. Note that H2v2=kH3v3 is satisfied by IA and n1 represents Gaussian noise that is added to a reception signal in a reception device and whose variance is denoted by σ2.
[Math. 13]
y
1
=H
1
v
1
x
1
+H
2
v
2
x
2
+H
3
v
3
x
3
+n
1
=H
1
v
1
x
1
+H
3
v
3(kx2+x3)+n1 (18)
In contrast, in the case where a channel matrix H is changed to H′ and a CSI error occurs, H2′v2≠kH3′v3 is satisfied and the reception signal y1 is expressed as the following equation.
[Math. 14]
y
1
=H′
1
v
1
x
1
+H′
2
v
2
x
2
+H′
3
v
3
x
3
+n
1 (14)
Here, x1 is a desired signal for the reception device 1. Thus, the reception signal y1 expressed as Equation (14) is the sum of the desired signal, two interference signals H2′v2x2 and H3′v3x3, and the thermal noise n1. A reception weight vector u for minimizing the mean square error between the reception signal y1 and the desired signal x1 may be obtained by solving the following equation.
Note that the first equation of Equation (15) indicates that a reception weight vector u is obtained with which the mean square norm (E(c) represents the average of c) of an error ε is minimized, the error ε is obtained between a result obtained by multiplying a reception signal and the reception weight vector u together and the desired signal. Here, assuming that power of each of the transmission signals x1, x2, and x3 is one, the reception weight vector u satisfying Equation (15) is expressed as the following equation.
[Math. 16]
u=(H′1v1)H{(H′1v1)(H′1v1)H+(H′2v2)(H′2v2)H+(H′3v3)(H′3v3)H+σ2I}−1 (16)
In this way, in a system in which IA is used, a reception weight vector for minimizing the mean square error about a desired signal may be calculated in accordance with each of the equivalent channel vectors of the desired signal and interference signals. The degradation of characteristics due to the effects of a CSI error may be reduced by multiplying a reception signal and this reception weight vector together. Note that, here, it is assumed that the power of each of the transmission signals x1, x2, and x3 is one, and consequently, σ2 and a unit matrix are multiplied together in Equation (16). However, in general, the inverse of SNR and a unit matrix are multiplied together.
The reception weight vector expressed as Equation (16) is a general MMSE reception weight vector in the case where one desired signal and two interference signals arrive. As illustrated in
However, in the IA used in the present invention, transmission weight vectors used in transmission devices are controlled such that the equivalent channel vectors of interference signals are brought into alignment at the time of reception, that is, such that the interference signals are easily eliminated on the reception side (here, such that H2v2=kH3v3 is satisfied). Thus, under circumstances in which a CSI error is not so large, the correlation between H2′v2 and H3′v3 is significantly high. This may be considered to be under circumstances in which H2′v2≅kH3′v3 is satisfied although the vectors are not completely brought into alignment. Under such circumstances, even in the case where the number of incoming signals is greater than then number of receive antennas, it is considered that the degree of freedom is not completely insufficient. Thus, it is possible to separate the desired signal from the interference signals by using the reception weight vector expressed as Equation (16) and to extract the desired signal. Thus, in the case where interference signals are not completely eliminated due to a CSI error even though the interference signals are controlled so as to be easily eliminated on the reception side, a desired signal may be extracted by using a MMSE reception weight vector as expressed by Equation (16) and a special effect is obtained, which is not obtained in a normal system.
A reception device that uses such a reception weight vector may be realized by using the same structure as the reception device illustrated in
The transmission devices according to the present embodiment may also be realized by using the same structure as the transmission device illustrated in
Moreover, pilot signals may have the same structure as the pilot signals illustrated in
Moreover, in the system illustrated in
In this way, in the case where a part of the interference signals is regarded as a desired signal and a reception weight vector is calculated, a vector regarded as a desired signal and a vector regarded as an interference signal may be obtained as follows. This is a method in which since the correlation between equivalent channel vectors H2′v2 and H3′v3 of two interference signals to which IA is applied is significantly high, either one of the vectors is divided into a vector projected onto the other vector and a vector orthogonal to the vector, and the projected vector and the other vector are regarded as an equivalent channel vector of a desired signal and a vector orthogonal to the other vector is regarded as an equivalent channel vector of an interference signal. This method will be described with reference to
Moreover, the equivalent channel matrix may be Heq=[H1′v1 H3′v3] and, about the vector p, power conversion of a signal may also be taken into consideration. This indicates that the power of a signal received via an equivalent channel having H3′v3 is treated as 1+a2 since p=aH3′v3. The reception weight vector in this case is determined to be the first row vector of HeqH{HeqΣHeqH+qqH+σ2I}−1. Note that Σ is a diagonal matrix whose diagonal elements are [11+a2].
As described above, the reception weight vector is calculated by regarding a part of the interference signals as a desired signal, and the degradation of reception characteristics due to the effects of a CSI error may be reduced also by using the calculated reception weight vector.
Furthermore, as described also in the first embodiment, a vector at the midpoint of equivalent channel vectors of interference signals is calculated and a reception weight vector may be calculated using the vector at the midpoint. Specifically, Heq is expressed as Heq=[H1′v1 (H2′v2+H3′v3)/2] and the reception weight vector is determined to be the first row vector of HeqH{HeqΣHeqH+σ2I}−1. Note that Σ is a diagonal matrix whose diagonal elements are [1 2]. Moreover, a sum vector is calculated and a reception weight vector may also be calculated not using the vector at the midpoint but using the sum vector. In this case, Heq is expressed as Heq=[H1v1 H2′v2+H3′v3] and the reception weight vector is determined to be the first row vector of HeqH{HeqHeqH+σ2I}−1. In this way, methods in which a vector at the midpoint of equivalent channel vectors of interference signals or a sum vector is used to calculate a reception weight vector are significantly effective as methods for reducing degradation of characteristics due to the effects of a CSI error, in the case where a leading cause that makes a CSI error occur is noise added to pilot signals in a reception device.
Moreover, the system illustrated in
Furthermore, as illustrated in
The above-described two embodiments represent cases where a transmission device has a plurality of transmit antennas, precoding is performed with respect to the transmit antennas, a reception device has a plurality of receive antennas, and a desired signal is extracted by multiplying a signal received by the plurality of receive antennas and a reception weight vector together. This indicates that IA, which is used in the present invention, is performed by using a plurality of space resources (antennas). However, the methods for reducing a CSI error according to the present invention are not limited to those in which IA is performed by using a plurality of space resources and may also be applied to those in which IA is performed by using a plurality of time resources or frequency resources. For example, precoding for one data signal is performed with respect to a plurality of sub-carriers in a system in which multicarrier transmission is performed. Even in such a case, similarly to as in the case where a plurality of space resources are used, a desired signal may be extracted even under the circumstances in which a CSI error occurs, by estimating an equivalent channel vector on a precoding-by-precoding basis, by calculating a reception weight vector as expressed by Equation (16), and by multiplying a reception signal and the reception weight vector together on a precoding-by-precoding basis. Note that, similarly to as in the above-described embodiments, the number of interference signals (undesired signals) controlled on a transmission side by using IA so as to bring the equivalent channel vectors at the time of reception into alignment, in other words, the number of interference signals to be received whose equivalent channel vectors are not brought into alignment due to the effects of a CSI error is greater than or equal to the degree of freedom determined by a plurality of resources.
Moreover, the above-described two embodiments describe basic cases where devices on a transmission side are controlled so as to collaborate with one another such that the (vector) directions of the equivalent channel vectors of interference components are brought into alignment at the time of reception, the interference components coming from a plurality of transmission devices, which are the interference sources. However, in IA, it is not always necessary to bring the directions of equivalent channel vectors of interference components into alignment. This is because interference is eliminated in the case where equivalent channel vectors of interference components are controlled so as to be orthogonal to a reception weight vector. As in the above-described embodiments, for example, equivalent channel vectors of interference signals do not have to satisfy H2v2=kH3v3. In this way, in IA in the case where the directions of the equivalent channel vectors are not completely brought into alignment, the following equation needs to be satisfied.
[Math. 17]
u
i
H
ij
v
j=0 i≠j
rank(uiHiivi)=dt (17)
Here, u represents a reception weight vector, v represents a transmission weight vector, H represents a channel matrix, and d is a positive integer other than zero. The first equation of Equation (17) indicates that a signal destined for a reception device j and received by the reception device i becomes zero after the signal and a reception weight vector are multiplied together, that is, interference is eliminated. Moreover, the second equation of Equation (17) indicates that the rank (also called the stream) of a signal that is received by the reception device and that is obtained by multiplying a signal destined for the reception device i and a reception weight vector together is di, that is, a desired signal is received without being eliminated. Even though the directions of equivalent channel vectors of interference components are not brought into alignment at the time of reception, interference components may be eliminated and a desired signal may be extracted by calculating transmission weight vectors v and a reception weight vector u that satisfy the relationships indicated by Equation (17), the number of the interference components being greater than or equal to the degree of freedom. The methods for calculating such transmission weight vectors and reception weight vectors are especially effective when there are three or more transmission devices as illustrated in
Even in the case where such control is performed, under the circumstances in which a CSI error occurs, it is difficult to effectively eliminate interference even when transmission weight vectors and reception weight vectors calculated by performing calculation repeatedly in the central control section are used. Thus, in each reception device, as expressed by Equation (9) or Equation (11), the effects of interference may be minimized by using a reception weight vector obtained by performing SVD on a matrix in which equivalent channel vectors of interference components are arranged, the interference components having been affected by changes in channels. Moreover, as a result of using a reception weight vector as expressed by Equation (16) obtained by solving Equation (15), a desired signal may be extracted considering not only interference but also noise.
Equation (16) represents a normal MMSE reception weight vector in the case where one desired signal and two interference signals arrive. The present invention is targeted at circumstances in which the degree of freedom is insufficient to eliminate interference and to extract a desired signal. Thus, even when the reception weight vector expressed as Equation (16) is used, it is difficult to appropriately extract a desired signal in a normal system other than IA. However, as described above, each of the equivalent channel vectors of interference signals is controlled so as to be orthogonal to a reception weight vector and to be easily eliminated on the reception side in a system in which IA is used. Thus, even when a deviation is generated for them due to a CSI error, the effects of error may be reduced and a desired signal may be extracted by using Equation (16).
Moreover, a program according to the present invention, the program being operated in a terminal device and a base station device, is a program (a program that causes a computer to realize functions) for controlling a CPU and the like such that functions of the above-described embodiments according to the present invention are realized. The information handled in these devices is stored temporarily in a RAM when being processed. Thereafter, the information is stored in various ROMs or HDDs, and is read, modified, or written as necessary by a CPU. Any of semiconductor medium (for example, ROMs, non-volatile memory cards, and the like), optical recording medium (for example, DVDs, MOs, MDs, CDs, BDs, and the like), magnetic recording medium (for example, magnetic tapes, flexible disks, and the like), and the like may be used as a recording medium in which the program is stored. There are cases where the functions of the above-described embodiment are realized by executing a loaded program. There are also cases where the functions of the present invention may be realized by performing processing together with an operating system or other application programs and the like in accordance with the instructions of the loaded program, which is being executed.
In the case where the program is put into the market, the program may be stored in portable recording mediums and then distributed or may be transferred to a server computer connected via a network such as the Internet. In this case, the memory device of the server computer is also included in the present invention. Moreover, a part of or all of terminal devices and base station devices at the above-described embodiments may be typically realized as LSI circuits, which are integrated circuits. The function blocks of the terminal devices and base station devices may be individually realized as a processor or a part of or all of them may be integrated to form a processer. In addition, the method for integrating circuits is not limited to LSI and may be realized by using dedicated circuits or general-purpose processors. In the case where a circuit-integration technology as an alternative to LSI is developed as semiconductor technology progresses, circuits may be integrated by using the circuit-integration technology.
As described above, the embodiments of the present invention have been described with reference to the drawings; however, specific structures are not limited to these embodiments. Other designs and the like that are not depart from the gist of this invention are included in the claimed inventions.
The present invention is applicable to a communication device.
1-1 . . . transmission device 1, 1-2 . . . transmission device 2, 3-1 . . . reception device 1, 3-2 . . . reception device 2, 10 . . . upper layer, 11 . . . modulation unit, 12 . . . transmission weight multiplication unit, 13-1 . . . first D/A unit, 13-2 . . . second D/A unit, 14-1 . . . first wireless communication unit, 14-2 . . . second wireless communication unit, 15-1, 15-2 . . . antenna, 16 . . . pilot signal generation unit, 17 . . . transmission weight calculation unit, 18 . . . reception unit, 19 . . . A/D unit, 20 . . . third wireless communication unit, 21 . . . antenna, 30-1 to 30-3 . . . antenna, 31-1 to 31-3 . . . wireless communication unit, 32-1 to 32-3 . . . A/D unit, 33 . . . signal separation unit, 34 . . . reception weight multiplication unit, 35 . . . demodulation unit, 36 . . . upper layer, 37 . . . channel estimation unit, 38 . . . reception weight calculation unit, 39 . . . transmission unit, 40 . . . D/A unit, 41 . . . wireless communication unit, 42 . . . antenna.
The entire contents of all the publications, patents, and patent applications cited in the present specification are incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-003047 | Jan 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/079892 | 12/22/2011 | WO | 00 | 7/10/2013 |
